You have LibreOffice and all the Mozilla stuff available for OSX, so your basic productivity needs are covered. A Google on "Macintosh freeware" should give you some places to start. I know MajorGeeks, Tucows, Opensourcemac, FreeMacware, and a bunch of others will all be there.
Might want to look at available free software before you commit to a build.
Addendum: unless you're fascinated at the thought of building a H-Mac why bother? For the same money and effort you could do up a very sweet home server. Just a thought...